A passive optical network (PON) consists of a central office node (also called an optical line terminal (OLT)), one or ore subscriber nodes (also called optical network units (ONUs) or sometimes optical network terminations (ONTs)), and the fibers and splitters between them (called an optical distribution network (ODN)). One type of PON is a wavelength division multiplexing (WDM) PON, which multiplexes a number of optical carrier signals onto a single optical fiber by using different wavelengths (colors) of laser light.
A WDM PON may or may not include wavelength selectivity in the structure of the ODN itself, for example, by the incorporation of arrayed waveguides (AWGs), each of whose physical ports selects one, or possibly two, specific wavelengths. If the ODN is wavelength-selective, the ONU and OLT are constrained by their physical connectivity to use one, or possibly two, specific wavelengths.
It is also possible for the ODN to not be wavelength selective, in which case the possibility is open for the OLT and ONU to communicate on any of a plurality of wavelengths.
WDM PONs may be very heterogeneous. For example, wavelength channels can have different data rates and use different protocols; e.g., 1 and 10 Gigabit Ethernet; 1.25, 2.5, 6 and 10 G common public radio interface (CPRI), and other services, modulation formats and protocols. For cost and technical reasons, both OLT ports and ONUs may be able to handle only one or at most a subset of these variants. Moreover, wavelength channels ay be provided by different service providers, each of which may assign wavelength channels either in a pre-assigned static way, or dynamically. Some channels may use error correction/encryption and some may not. Some channels may be put into sleep mode, which is pertinent to the present invention because at a given time, there may be no signal present on a wavelength that is nevertheless allocated and in use.